1. Field of the Invention
The present invention relates generally to an electrostatic image-forming system such as duplicating machines, laser beam printers and facsimile units using normal paper as print media. More particularly, it relates to a second cassette feeder for an electrostatic image-forming system which may be optionally attached to increase the sheet stacking capacity.
2. Description of the Related Art
In a conventional electrostatic image-forming system, if a user sends a command to print, a charger 42 uniformly charges an outer surface of a photoconductive drum 50 rotating at a predetermined velocity. A laser scanner unit generates a pixel laser beam corresponding to a time-serial electrical pixel signal of image data input from an original image photoelectrical reading unit (not illustrated), and then scans the surface of the photoconductive drum 50 with the laser beam through the mirror and an exposure window so that an electrostatic latent image corresponding to the image data is formed on the photoconductive drum 50.
The charged latent image areas attract and hold toner supplied by a developing unit 44 to be visualized as a toner image, and a paper pickup roller 45 picks up a sheet of paper 46 (print media) stacked in a paper cassette. The toner image that has been formed on the surface of the photoconductive drum 50 is transferred to the paper 46 by a transfer charger 48. At this point, the paper 46 adheres closely to the outer surface of the photoconductive drum 50 by electrostatic force, and the paper becomes separated from the surface of the photo-conductive drum 50 by a separator 49. The paper 46 is then conveyed between a heating roller 51 and a compression roller 52 of a fixing unit, and thereby the toner image is fixed in such a manner that an image-formed printer output can be achieved.
After toner image transfer, a cleaner 40 removes a residual toner on the surface of the photoconductive drum 50, and a pre-erase lamp array 43 eliminates the toner image remained on the surface of the photoconductive drum 50.
A paper cassette of the above-described image-forming system may have 150 to 200 sheet capacity, and a second cassette feeder (SCF) 55a with a special driving mechanism is optionally provided in a main body of the image-forming system in order to increase the capacity of the paper cassette. This SCF 55a is mounted under a main cassette feeder 55, and a paper pickup roller 45a that is used to pick up a sheet of paper 46a stacked in the SCF 55a cannot deliver the paper 46a to a registration roller 39 of the main body. Thus, the SCF 55a needs a relay roller 56 and a roller opening device (e.g. spring) used to separate the rollers from each other in case that there is a paper jam in the relay roller 56.
As mentioned above, it is difficult for the conventional SCF to deliver a sheet of paper to the registration roller by one rotation of the paper pickup roller, and the SCF should have the relay roller, which increases the size of the SCF. Besides, the SCF needs a special jam-removal mechanism to thereby make the image-forming system complicated.